1. Field of the Invention
The present invention relates to an information recording/reproducing device.
2. Description of the Related Art
In recent years, small portable apparatuses have spread worldwide, and simultaneously, demands of the nonvolatile memory with small size and large capacity expand rapidly with extensive progress of a high-speed information transmission network. Above all, in particular, NAND-type flash memories and small-size hard disk drives (HDDs) have evolved in rapid recording densities, resulting in forming great markets.
However, it is pointed out that both the NAND-type flash memories and small-size HDDs will reach the limit of recording density soon. With respect to the NAND-type flash memory, a processing cost is increased by a reduction of the minimum width of a conductive line. And with respect to the small-size HDD, it is not possible to sufficiently reserve tracking accuracy.
Under this situation, some ideas of new memories are provided to exceed the limit of the recording density.
For instance, phase-change RAM (PRAM) uses a material as a recording material capable of taking two states of an amorphous state (on) and a crystalline state (off) to adopt the principle in which these two states correspond to binary 0 and 1 in order to record data.
Concerning the writing/erasing, for instance, the amorphous state is formed by applying a large electric power pulse to the recording material, while the crystalline state is formed by applying a small electric power pulse to the recording material.
The reading is performed by measuring an electrical resistance of the recording material while causing a small read current to flow into the recording material, the read current being too small to cause the writing/erasing. The resistance of the recording material in the amorphous state is larger than the resistance of the recording material in the crystalline state, and its difference is about 103.
The maximum feature of the PRAM lies in that the PRAM can operate even though the element size is reduced to about 10 nm. In this case, since the recording density of approximately 10 Tera bit per square inch (Tbpsi) can be realized, so that the PRAM is one of candidates for high-density recording (for instance, refer to T. Gotoh, K. Sugawara and K. Tanaka, Jpn. J. Appl. Phys., 43, 6B, 2004, L818).
Further, there has been reported a new memory, different from PRAM, having an operation principle which is very similar to that of the PRAM (for instance, refer to A. Sawa, T. Fuji, M. Kawasaki and Y. Tokura, Appl. Phys. Lett., 85, 18, 4073 (2004)).
According to the report, a typical embodiment of the recording material for recording the data is nickel oxide, and the large electric power pulse and the small electric power pulse are used for the writing/erasing like the PRAM. In this case, there is reported an advantage that the power consumption decreases at the time of the writing/erasing compared with the PRAM.
Until now, an operation mechanism of the new memory has not yet been found, but reproducibility is confirmed, and thus it is another candidate for high-density recording. Further, some groups try to elucidate the operation mechanism.
In addition to this, there is proposed a MEMS memory using micro-electromechanical system (MEMS) technology (for instance, refer to P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. During, B. Gotsmann, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz and G. K. Binnig, IEEE Trans. Nanotechnology 1, 39 [2002]).
In particular, in the MEMS memory having a structure in which a plurality of array shaped cantilevers are confronted with the recording medium to which an organic material is applied, a probe at a distal end of the cantilever comes into contact with the recording medium with a proper pressure.
The writing is performed by controlling temperature of a heater added to the probe selectively. That is, when raising the temperature of the heater, the recording medium is softened, the probe is sunk into the recording medium, and thus depression is formed on the recording medium.
The reading is performed by scanning the probe with respect to the surface of the recording medium while causing a current to flow through the probe, the current being too small to soften the recording medium. When the probe is fallen into the depression of the recording medium, the temperature of the probe decreases, so that the resistance of the heater rises. As a result, it is possible to sense the data by reading the change of resistance.
The maximum feature of such a MEMS memory lies in that it is possible to dramatically improve the recording density because it is not necessary to provide a wiring in each recording part for recording the bit data. At the present state, the recording density of about 1 Tbpsi has been already achieved (for instance, refer to P. Vettiger, T. Albrecht, M. Despont, U. Drechsler, U. During, B. Gotsmann, D. Jubin, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz, D. Wiesmann and G. K. Binnig, P. Bachtold, G. Cherubini, C. Hagleitner, T. Loeliger, A. Pantazi, H. Pozidis and E. Eleftheriou, in Technical Digest, IEDM03 pp. 763-766).
Further, recently, approaches of achieving large improvement of the power consumption, the recording density and the working speed are performed while combining MEMS technology and new recording principle.
For instance, proposed is a system in which there is provided a ferroelectric layer on the recording medium, and the recording of the data is performed by causing dielectric polarization in the ferroelectric layer by applying a voltage to the recording medium. According to the system, there is theoretical prediction that it is possible to approach intervals (recording minimum unit) between the recording portions for recording the bit data to unit cell level of the crystal.
If the recording minimum unit becomes one unit cell of the crystal of the ferroelectric layer, the recording density becomes a very large value of about 4 Peta bit per square inch (Pbpsi).
However, such a ferroelectric recording MEMS memory has not been realized yet although its principle has been conventionally known.
The largest reason is that an electric field radiated from the recording medium is shielded by an ion in air. That is, since the electric field from the recording medium can not be detected, it is not possible to perform reading.
Further, another reason is that, when lattice defect exists in the crystal, electric charges are shielded such that electric charges caused by the lattice defect move to the recording part.
The former problem of the electric field shielding caused by the ion in air has been resolved by a proposal of the read system using a scanning nonlinear dielectric microscope (SNDM) recently, and thus the new memory has made a great progress toward the practical use (for instance, refer to A. Onoue, S. Hashimoto, Y. Chu, Mat. Sci. Eng. B120, 130 [2005]).